The whistle-blower who exposed former Greens senator Scott Ludlam’s failure to renounce his dual New Zealand and Australian citizenship has been revealed.

Mr Ludlam stepped down from federal parliament on Friday after acknowledging he was “ineligible” to sit in the Senate and has been for nine year tenure.

Perth barrister Dr John Cameron told the Weekend Australian his application to the New Zealand Department of Internal Affairs to search its register of citizens was “not driven by political ideology”.

“I did this as a citizen, not as a lawyer with a keen interest in the Constitution,” Dr Cameron told the newspaper.

Under Section 44 of the constitution, potential candidates to the Parliament of Australia are disqualified if they hold dual or plural citizenship.

Dr Cameron said he searched the NZ register in relation to Mr Ludlam and Senator Derryn Hinch.

He later received a certificate that showed the former was still a citizen, while the latter had renounced his citizenship before the last election.

“I expected the human headline may not have done it and Mr Ludlam would have done it, but it was the other way around,” Dr Cameron said.

“I sent the certificate to the clerk of the Senate, however out of courtesy a few days ago I provided Senator Ludlam with a copy of the certificate. This is not driven by political ideology.”

At a press conference Mr Ludlam apologised for his mistake and said he should have checked his citizenship status when he was first nominated for pre-selection in 2006.

“It was brought to our attention a week ago, while I was still overseas, by someone who had done the digging for whatever reason,” Mr Ludlam said.

“It took a couple of days to confirm with the New Zealand High Commission that was the case.”

Born in New Zealand in 1970, Mr Ludlam left the country to settle in Australia when he was three years old.

“This town is home. I have been here since 1978. It never crossed my mind that citizenship might be something that sticks to you in that way,” Mr Ludlam said.

The former Greens co-deputy leader may have to repay the Commonwealth $1.6 million in salary received during his time as an elected senator.

Mr Ludlam said he did not have the means to pay back the money and was confident it would not come to that.

“It’s going to be millions of dollars and my total assets amount to a fast computer and some nice shoes,” he said.

Mr Ludlam’s position will be filed by a recount of ballot papers from the 2016 election, leading to the possible election of 22-year-old Jordan Steel-John who was third on the Greens senate ticket.